Inkjet recording process has been in rapid progress in various fields, because high-definition images can be recorded in a relatively simple apparatus. Applications of such process have also been diversified, and thus various recording media or inks suitable for respective applications are selected. In recent years, application of the inkjet recording process to industrial fields is under study, and in particular, the development is advanced to provide printers having performances satisfying the requirements of an on-demand type application of carrying out facilitated printing.
The process and the ink used for recording are also studied from various points. For example, an inkjet recording inks, that are hardened when they are exposed to an ultraviolet ray, are in development. Such an ink has a significantly shortened image-fixing period and improved fixing efficiency, compared to inks that should be air-dried or left for penetration into the substrate. However, there is still a need for further improvement of hardening velocity and reduction of hardening energy. It is important in industrial application that the image is brilliant and highly vivid, and also, that a great number of prints are formed in an extremely short period of time at low energy consumption.
As to the UV-hardening inks as described above, radical-hardening inks mainly based on acrylic compositions are studied (see e.g., Japanese Patent No. 3619778 and JP-T-2000-504778 (“JP-T” means published Japanese translation of PCT application). Further, reduction in the size of pigment particles contained in an ink is attempted for improvement of image brilliance, but the diameter of the particles is 120 nm or more, even if the smallest one, for example, specifically disclosed in JP-A-2005-179506 (“JP-A” means unexamined published Japanese patent application). On the other hand, radical-polymerization inks have a problem that the hardening efficiency is reduced by polymerization inhibition caused by oxygen.
In addition, as a colorant for color filter, colored resin-fine particles having an average diameter of 100 nm or less, which contains a water-insoluble colorant, are disclosed (see JP-A-2008-150507). The resin component of the colored resin-fine particles includes a hydrophilic resin and a hydrophobic resin, and the average diameter of the water-insoluble colorant is 50 nm or less. By the use of such particles, it is reported that improvement can be shown in color-developing efficiency and image fastness of the resulting printed article. However, a step of dispersing the fine water-insoluble colorant particles as resin fine-particles require extremely long period and increased energy. Further, bulky particles may be unwillingly contained due to less dispersion ability of the water-insoluble colorant particles. Further, use of the particles in a nonaqueous ink makes dispersion more difficult, and it is not clear whether the favorable dispersion state can be achieved.
On the other hand, as an aqueous ink which does not demand hardening by ultraviolet ray or the like, it is disclosed that an ink containing pigment particles made in a smaller diameter and in a narrower particle diameter distribution for improving a bronzing property, a glossiness property, and others (see, e.g., JP-A-2003-113341 and JP-A-2003-128955). When used in such an aqueous ink, for example, the pigment fine particles having a particle diameter of 30 nm or less, as disclosed in the Patent Documents above, can be utilized in a stabilized state. However, it is not obvious whether such a nonaqueous hardening ink can show favorable ink properties, while preserving its light resistance sufficiently, to be used as an ink which is fixed to be an image by irradiation of an activating energy ray such as ultraviolet ray or by heating.